In an important system such as a server, a power supply device equipped with a redundant power supply unit or a battery unit which can perform electric power supply at the time of power failure is used in order to raise reliability. A redundant power supply unit is a power supply unit which supplies electric power instead of a working power supply unit in case abnormality has occurred in the working power supply unit which supplies electric power at the time of normal operation.
In patent document 1, a power supply device which supplies power supply to a computer including a battery module to supply electric power to a main power supply line at the time of occurrence of power failure is disclosed. When power failure is not occurred, the power supply device described in patent document 1 charges the battery module by a standby power generation unit which supplies electric power for power supply management to the system at all times including a time when electric power is powered off. Also, when power failure occurs, the power supply device described in patent document 1 supplies electric power from the charged battery module.
In patent document 2, a battery charging method for charging a battery by a charger connected in parallel to the battery is disclosed. The battery charging method described in patent document 2 makes an electric power detector monitor load power of charging of a battery, makes a sequencer calculate a minimal number of chargers necessary to supply the load power, makes calculated number of chargers operate, and charges the battery.